ADU rules in Alpine

Ordinance 2026-16, adopted by the City Council on September 8, 2026 and not yet in the online code, adds §3.14.030: "Detached accessory dwelling units may be a permitted use" where single-family dwellings are a permitted use in the zone. The ordinance sets no minimum lot size; instead the allowed size of the unit grows with the lot, from 800 sq ft of living area on lots under 10,000 sq ft. The unit cannot push total building coverage of all permanent structures past 35%, 30%, 25% or 22% of the lot (by lot size), so a lot already near that limit may not qualify. Setbacks follow the accessory-building rules for the zone, height is capped at the accessory-building limit and never above 25 feet, the design must be consistent with the main home, and a legally built accessory structure such as a garage may be converted. Utah Code 10-21-304, in force since October 1, 2026, requires Alpine to permit detached ADUs on single-family lots of 11,000 sq ft or more; Alpine's rule also covers smaller lots.

Internal ADUs (a unit within and part of the main home with its own cooking, sleeping and sanitation facilities) remain allowed in owner-occupied single-unit detached dwellings, with at least one separate outside entrance on the side or rear of the home. The zone use lists make ADUs a permitted use in the TR-10,000, CR-20,000, CR-40,000 and CE-5 zones and a conditional use in CE-50; the B-C zone lists accessory apartments as a conditional use, and the Senior Housing Overlay Zone prohibits them. Ordinance 2026-16 moves these internal-unit rules from §3.14.010 to a new §3.14.020 with the same core requirements, adding a 90-day rental minimum, annual registration, and a one-ADU-per-lot rule.

Under new §3.14.030 a detached ADU must have at least 300 sq ft of living area and no more than 800 sq ft on lots under 10,000 sq ft, 900 sq ft on lots of 10,001 to 20,000 sq ft, 1,000 sq ft on lots of 20,001 to 40,000 sq ft, and 1,200 sq ft on lots over 40,000 sq ft. Living area counts all enclosed rooms, hallways, closets and storage but not garages. The unit is also limited by a lot-coverage cap on all permanent structures (35%, 30%, 25% or 22% by the same lot-size tiers), and the Council's adopting motion applies whichever of the two limits is smaller. Internal ADUs have no square-footage cap; they only have to meet the size and access requirements of the International Residential and Building Codes.

An internal ADU is allowed only in an owner-occupied home. For a detached ADU the owner may live in either unit: "Detached accessory dwelling units shall be owner-occupied or be located on a property that has an owner-occupied single-family dwelling" (Utah Code 10-21-304(4)(c) allows this). Both rules waive owner occupancy during a documented temporary absence of 3 years or less (job assignment, sabbatical, military or voluntary service) or a move into a hospital or care facility, if the owner applies before leaving and lived in the home for at least 1 year first. An owner occupant (§3.01.110) holds 50 percent or more of the recorded ownership, or is a trustor of a family estate-planning trust that holds title, and lives in the home as a primary residence; if several people own the property and not all of them live there, none of them counts unless additional conditions are met.

A home with an internal ADU must provide at least four off-street spaces in total, counting garage and driveway spaces, with at least one designated for the ADU. A detached ADU needs on-site parking on top of the spaces required for the house: one space for a unit of 650 sq ft or less, and two for a larger unit. Those numbers stay within the limits of Utah Code 10-21-304(3)(b) and (c), which allow no more than two spaces for a detached unit of 650 sq ft or more and no more than one for a smaller unit.

In the TR-10,000, CR-20,000, CR-40,000 and CE-5 zones an ADU is a permitted use. The owner gets an Accessory Dwelling Unit permit from the Building Department, plus a building permit for any construction, and renews the ADU registration every year (the city's October 2026 application lists a $101 registration and inspection fee). In the CE-50 zone ADUs are a conditional use. Detached ADUs are limited to zones where a single-family home is itself a permitted use, which excludes CE-50, where a single-family home is a conditional use. Utah Code 10-21-304(3)(a) bars a conditional use permit for a detached ADU in a primarily residential zone.

Ordinance 2026-16 bars renting an internal or a detached ADU for a shorter term: each "may not be rented or offered for rent for a period of less than ninety (90) consecutive days". Separately, each residential zone's special provisions separately ban short-term rentals of 30 days or less. Every year the owner must file an ADU registration and rental license renewal, and pays the rental license fee if the unit is rented. A detached ADU may be occupied by one person or by a family as the code defines it. As amended at adoption, a lot with an internal unit may also add a detached ADU, but only one of the two may be used as an accessory dwelling; the city's October 2026 application asks the owner of a second unit to certify that it will not be rented. Utility meters must be shared with the main home and held in the owner's name.